38. Count and Say
题目
The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...
1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.
Given an integer n, generate the nth sequence.
Note: The sequence of integers will be represented as a string.
思路
一轮一轮reduce过去, 数一下个数写进去
代码
# @param {Integer} n # @return {String} def count_and_say(n) (1..n-1).inject("1") do |res,_index| to_next(res) end end def to_next (str, num = 0, cur = nil, res=[]) str.each_char do |c| if cur == c num = num + 1 else res.push(num.to_s, cur) if cur != nil cur = c num = 1 end end res.push(num.to_s,cur) if cur != nil and num != 0 res.join end